1. Field of the Invention
The present invention relates to an image forming apparatus and method which corrects distortion of an image, such as skew and bow, and magnification power, and then forms an image.
2. Description of the Related Art
When an image is formed (printed) by using a Xerography basis printer (image forming apparatus) or the like, distortion such as skew and bow, and misregistration such as variation of magnification power sometimes appear in an output image.
In JP-A-8-146317 (article 1), Japanese patent No. 2749367 (article 2) and the like, there is disclosed a method for correcting such a misregistration by changing an optical path of a light beam used for forming an image on a photo-receptor.
In JP-A-8-218400 (article 3) and the like, there is disclosed a method of correcting the magnification power by modifying a video clock used for forming an image on the photo-receptor.
In JP-A8-85236 (article 4) and JP-A-2000-112206 (article 5), there is disclosed in a method of correcting a color misregistration, which is caused in a color printing, by the image processing.
To realize the method as disclosed in the articles 1 to 3, it is necessary to provide a hardware component exclusively used for the correction in an optical system of the printer.
Accordingly, where the method disclosed in any of those articles is employed, the apparatus size is likely to be large, and cost problem is also likely to arise.
The skew correction mutually affects the bow correction. Accordingly, when one distortion is corrected, the other distortion further progresses.
On those corrections, there is a hardware performance limit, and accordingly, their accuracy and performance are always below the limit.
In a printer for printing by use of the screen, when the method of the article 4 or 5 is applied to the image having undergone the screen process, the pixels for the magnification correction interfere with the screen used for printing. Another misregistration, such as interference fringes, possibly appears in the output image.
When to avoid such an interference, the method is applied to the image before it is screen processed, a misregistration of the screen structure per se is left sometimes.